1. Field of the Invention
The invention is directed to flame retardant electrolyte solutions for rechargeable lithium batteries, and to rechargeable lithium batteries including the electrolyte solutions.
2. Description of the Related Art
Lithium rechargeable batteries have recently drawn attention as power sources for small, portable electronic devices. They use organic electrolyte solutions, yielding twice the discharge voltage of conventional batteries using alkali aqueous solutions. Accordingly, lithium rechargeable batteries have high energy density.
Lithium-transition element composite oxides capable of intercalating lithium (such as LiCoO2, LiMn2O4, LiNiO2, LiNi1−xCoxO2 (0<x<1), and so on) have been used as the positive active materials of rechargeable lithium batteries.
Various carbon-based materials (such as artificial graphite, natural graphite, and hard carbon) capable of intercalating and deintercalating lithium ions have been used as the negative active materials for rechargeable lithium batteries.
A lithium salt dissolved in a carbonate-based solvent has been generally used as the electrolyte solution. Recently, in an effort to improve flame retardancy of the electrolyte solution, a mixture of cyclic and linear carbonate solvents and an additive amount of a phosphoric acid-based retardant has been suggested. However, the phosphoric acid-based retardant causes reductive decomposition during reaction of the negative electrode and the electrolyte solution, thereby decreasing the available capacity of the negative electrode and increasing cell resistance. As such, the phosphoric acid-based retardant suppresses the smooth intercalation of lithium ions.
Furthermore, if the phosphoric acid-based retardant is added in an excessive amount (i.e., in a solvent amount instead of in an additive amount), the cycle-life characteristics of the resulting battery is significantly decreased.